Circuit controller



Dec. 12, 1933. w. HOPP CIRCUIT CONTROLLER Filed April 5, 1950 Inventor\X/iLheLm P16 019 I ocw/ His Attorne g.

Patented Dec. 12, 1933 CIRCUIT CONTROLLER Wilhelm Hiipp, Heiligensee,Germany, assignor to General Electric Company, a corporation of New YorkApplication April 5, 1930, Serial No. 441,982, and in Germany July 11,1929 I 15 Claims.

My invention relates to switches and has for an object the provision ofan improved switch with thermal release which has large switchingcapacity and high switching speed. Though my invention is suitable for awide variety of application it is particularly suitable as a protectiveswitch for arc rectifiers or the like.

In the case of rectifiers with glass envelopes, protection againstoverload is ordinarily obtained by means of explosion-proof safetyfuses, because such devices provide the variable permissible switchingtime which is in agreement with the heating of the anode 1ead-in causedby varying overloads. With the customary overload current switch andeven a quick acting switch, the desired protection can not be obtainedbecause every mechanically operated switch has a definite operating timeof its own which is determined by the inertia of its moving parts. Inorder to increase the disconnecting speed, it is necessary to reduce themass of the movable parts of the switch to a minimum.

In the case of thermally released switches, quick action is most readilyobtained by direct heating of the thermal element, since indirectheating causes a delayed release. However, for heavy current serviceordinarily a special cutout switch is required which is controlledmechanically or electrically by a thermal time relay. In.

this case, the particular time required for operation of the excesscurrent switch depends on the heating of the time element as well as theretarded operation of the cutout switch, so that in the case of arcrectifiers, considerable damage may occur before the endangered circuitis interrupted. f

Compared with this, the improved switch embodying the present inventionhas a very short opening time and'at the same time large currentcontrolling capacity. According to the preferred form of the inventionthe movable switch actuating part consists of a 'bi-metallic vthermalelement formed as the main switch lever and traversed by the maincurrent. The bi-metallicelement carries the movable contact atits freeend. The thermal element itself is thus used'directly for effecting theinterruption of the current. In this case the relatively high contacttemperature occurring due to the heating of the bimetallic element isovercome by the use of non-oxidizing contact parts. Moreover, that thewelding together of the interrupting contacts is prevented to a veryconsiderable degree by means of an overcenter spring mechanism providinga suflicient contact pressure during the disconnecting movement, andwhich also insures the, immediate separation of the contacts with greatswitching speed. The further details of the invention may be bestexplained by means of the constructional example represented inthedrawing. I

In the accompanying drawing Fig. 1 is a side view partly in section of acircuit controller. em-. bodying the present invention; Fig. 2 is achart showing the variations in the operating forces occurring duringoperation of the circuit con- 05 troller; Fig. 3 is an enlarged viewshowing a'modiw fication of the operating electromagnet for'the switch;Fig. 4 is a view partly in section showing the detailed construction ofthe electromagnet armature and the thermal element; Fig. 5 is a top viewshowing the detailed construction of. the contact mechanism and Fig. 6shows a modi-. fication of the'switch operating electromagnet structure.

As'shown in Fig. 1 the bi-metallic strip 1' traversedby the main currentis firmly secured at its lower end, and at its upper end carries themain switch contact 2, whose mass is small. The result of this is thatthe temperature of the bimetallic element is transferred to the maincontact with hardly any diminution. Since for the safe protection ofelectric lines and machines the operating temperature required for thebimetallic strip fluctuates within the limits of 100-200" C., theoxidation temperature for customary copper contacts is considerablyexceeded. In order to overcome this difiiculty, the main moVablecontactor the cooperating contact piece is provided .with a silver insertion 3atthat point at which the contacts touchone another in the circuitclosing position. The cooperating contact 4, moreover, is not rigid, butis arranged so as to be yielding and is limited in its switching path bymeans of a stop 5. In order to keep the mass of the cooperating contactsmall, the current is con- 5 ducted to the same through a resilient band6. The opening movement of the main contact 2 is also limited by a fixedstop '7. The operating force of the cooperating contact spring 6 isproportioned to that of the bi-metallic strip 1. In order to effect aninstantaneous disconnection a powerful overcenter spring 8 is arrangedso that on the opposite sides of the dead center, or critical position,it exerts a greatly increasing additional force on the contact 2 toeither to close or to openthe same.

The operation of the forces of the mechanical system described,according to Fig. 1, can be seen from the diagram of Fig. 2. Thecharacteristic lines, that i to say, the curves of the forces exerted bythe resilient contact 1 and of the resilient cooperating contact 4, areapproximately straight lines. The characteristic of the force occasionedby the tension of the overcenter spring 8 is also practically'a straightline in the case of its displacement beyond the dead center position.The position and direction of these characteristic lines are so selectedaccording to the invention that the desired quick operation is broughtabout in the following manner.

The closed position of the contacts indicated in Fig. 1 is selected inthe diagram as the zero point, the movement of the switch elements beingrepresented along the abscissas and the forces on the switch elements bythe ordinates. The closing force acting to the left in Fig. 1 isreckoned as'positive and the opening force as negative.

The force at the contacts 2 and 4, due to the displacement of thethermal element, is represented by the line Br at normal temperature,but it will be seen that this force is zero in the closed position. Ifthe temperatureof the thermal element is increased above the normaltemperature by a predetermined amount the force at the contacts will berepresented by a straight line which is parallel to the line Br anddisplaced from it a predetermined amount. For example, assuming that thethermal element isunrestrained, the force at the contacts will berepresented by the line By and in the diagram this line corresponds tothe temperature required 'toopen the contacts. Upon reaching thereleasing temperature, however, the characteristic line of thebimetallic strip is displaced by the amount Bgo which represents thethermal expansion due to the heating producedby the load.

The characteristic line Z of the spring 8 (considered with respect tothe force exerted on contact 2) always cuts the zero force line in thedead center of the path of movement of contact 2 which distance isrepresented by the distance St. along the abscissa axis. In the zeroposition the force of the spring has the initial value represented bythe ordinate Z0. Finally there is further the characteristic line K ofthe counter contact 4. In the closed position the force on the contact 4is represented by the ordinate -Ko (minus because it represents anopening force which has been plotted above the abscissa). As shown, thisforce diminishes somewhat before the contact 4 is arrested by the stop5. The force at the distance Sic drops to zero. In order to mainvtainthe switch in the closed position, Zo must of course be greater thantheinitial force Ko of the counter contact. As shown, the contactpressure is' represented by the difference between the forces Z0 and K0.7 With increased heating of the bimetallic element a force is developedtending to open the contacts and counteracting the residual force-due toZ and K. When the force due to the thermal strip is such as thatrepresented by the line By the net force on the contact element is zeroand a very slight further increase will produce a net motion. Thecontacts are opened a distance So, large enough to interrupt the currentbut kept as small as possible to reduce stresses in the bimetallicelement as much as possible.

The overload switch fulfills the further requirement that when thebi-metallic strip cools no further automatic reclosure of the switchtakes place. If the reclosure of the switch is desired, it can beaccomplished without difficulty. For example, if S0 is made larger thana certain amount, the force of the bimetal tending to close the contacts(when cool) is less than that of the overcenter spring tending to keepthe contacts open. This arrangement would be used when it is desired tohave the switch remain open after being tripped. If automatic closing isdesired, So is made smaller so that Z-Br (representing the net force onthe contacts) has a positive value tending to close the contacts. Thecycle for the heating and. the cooling of the thermal element isindicated byv the plotted arrows.

. The present construction requires only a small operating force inorder to reclose the switch. Further, the entire system can be arrangedpractically without friction due to the absence of all complicatedintermediate members, such as levers, pawls and rods. Thus the necessaryforces for automatic operation are also small when the switch is to bereclosed or opened independently of the bimetallic strip. The new switchtherefore is particularly adapted for remote operation, since itsoperating magnets may be exceptionally small.

It can be seen from the diagram that the conditionsare fully illustratedfor-the special case mentioned. The variation of the value of thecurrent required for automatic operation of the switch can be obtainedin different ways, for example, by varying one or the other constant ofthe system. For convenient calibration a variation of the springconstants is desirable. The cooperating contact 4, for example, can havean additional spring 9, arranged so that the characteristic line resultsfrom the combined effect of the two springs 6 and 9. In addition tovarying the pressure by tensioning or untensioningthe spring 9, itsconstant can be adjusted by varying the' number of turns. The springconstant of the over-center spring 8' can be conveniently regulated bymeansof an adjusting nut 10. Accordingly, as its force is varied theposition of its characteristic curve Z is varied, since the latter mustalways pass through the dead center point St.

In the case of heavy short-circuits the temperature of the bi-metallicstrip increases in the same manner as in the case-of a safety fuse.Since there is no time for conducting away the heat on a suddenoverload, the electric energy causes a quick rise in the temperature ofthe thermal element, whereby the expansion takes place ven quickly. Toopen the switch it is only required to accelerate the small additionalmass of I 12' of the main current magnet 13 as a magnet armature. In thecase of switches for small current strengths, it will be understood thatthe resilient current conducting band 6 may consist of a magnetizablematerial, for example, steel, so that a special armature 11 isunnecessary.

The blow-out magnet 13 with coil 14 is used as main current magnetaccording to the invention. For obtaining a large switching capacity,contrary to the customary arrangement on excess current switchboards,this is not arranged above or behind the main contacts, but in front ofand somewhat below the same. A stationary auxiliary electrode 15 extendsover the blow-out coil and down the front side of the switch, so. thatthe interruption arc can extend on the front switch surface entirelyfree and unhindered. At

both sides special pole horns 16 are provided,

which are bent somewhat at right angles, and

embedded in a U-shaped recess, is covered almost completely byinsulating material. Only the upper part of the contact which is locatedin the blow-out field is exposed. In this way the switch is protected toconsiderable degree against the injurious effect of the arc gases in thecase of disconnection through short-circuit, against re-ignition, andagainst interpole sparking. The blowout arrangement is not builttogether with the contact, but is located at the front part of theswitch. In the open position of the switch, no voltage difference existsbetween the contact and the blow-out poles. The front pole-of theswitch, with the construction described, can without difficulty beentirely covered with insulating plates 17 since the operating mechanismand the connection for the individual switch poles (cross-bar 31 in Fig.4) are located at the lowest end at the greatest distance from the arc.

The total switching time ofa switch, as is known, is made up from itsown time and the duration of the arc. In order to avoid excessiveheating of the thermal element 1 while-the arc lasts, the stationaryhorn 15extends over the contact 2, so that after the occurrence of theare it is led over on to'the horn 15 in the shortest time. Thisarrangement, in this case serves the special 'purpose of preventingexcessive heating of the bimetallic element and thus shortens the timeinterval which is necessary before the reclosing of the switch can beaccomplished.

As-explained by means of the diagram Fig. 2, an automatic reclosing ofthe switch can be effected after the cooling of the bimetallic elementby an appropriate adjustment of the spring constants. In order to beable to effect a reclosure before the co aling occurs, the dead orcritical point position of the rocking lever 1 can be varied. For thispurpose the lower suspension point of the rocking spring 8 is mounted ina swing 21 which, as more clearly shown in Fig. 3, is rotatable aboutthe point 25 against the eflectof a stop 18 which is yielding in theswitch closing direction and which is pressed into its end position, bythe spring 19.

To operate the switch by hand only a small motion of the fixed end ofthe over-center spring is required. This can be shown in the diagram asa displacement of the line Z to the left or right. If Z is displaced adistance Sa to the left the force Z0 becomes equal to K0 and a positionof unstable equilibrium ensues and the switch opens. If the switch isclosed and Z is displaced to the right a distance Se the force of theovercenter spring is reduced to equal the force of the bimetallicelement and the switch snaps closed.

If the switch is to-be remotely operated then the swing'21 isformed as amagnet armature. At either side of this swing is arranged the closingand'opening magnets 22 and 23 respectively.

A particularly effective remote release is obtained f or the reason thatthe torque originating from the tilting spring 8 in the closingdirection is partly or entirely balanced byan auxiliary force, forexample vby the springs 24. This arrangement has no. influence on thecontact pressurebut to effect the opening of the switch only a smalljmovement of swing 21 is required. If

thejswinging armature 21 is further mounted on leaf springs-25 like aclock pendulum, so as to be free -from'friction, the sensitiveness canbe increased'to an extraordinary degree. This is of great-importancesince it often is desirable that the switch be controlled by means of anauxiliary sitive' to faults other than overload. 1 rangement of anauxiliary magnet armature '11 Fig. 1 on the counter contact 4 alreadybove gives without difliculty the unretarded rel se in the case ofexcess current. This is accomplished because the contact path Sk, as

can be seen in the diagram, is greater than the dead center position. Ifthe oscillating armature 21'has time to fall into the switch openingposition before the interruption arc has been .ex-'

tinguished, then the switch opening operation is ensured. If however thearc breaks, say, as a result of the existing counter E. M. F., then thecontact 2. springs back into the switch closing position since thetemperature of the bi-metallic strip 1 has not yet reached the releasingtemperature. This kind of release is found to be particularly effectivefor very high short-circuit currents. However an additional overloadrelease independent of the heating is desirable. Therefore on theblow-out magnet 13 additional pole pieces 27 are arranged which extendas far as the oscillating armature 21 and magnetize the section of thepath St which extends only to the latter independently of the releasingcoil 23.

Thus with the desired excess current the switch is moved positively intothe disconnected open circuit position. The releasing current strength.

can be adjusted by arranging a spring to exert a counter force on thereleasing armature 21 or by varying the gap between armature and magnetpole.

The described switch construction renders possible an extremely simplearrangement of a remote release, because the forces necessary for theoperation are relatively small, and particu-' larly because the movementof the operating mechanism is very small. In contra-distinction to thisthe known switches require complicated lever mechanisms and pawl lockingdevices. In accordance with the present invention the means for insuringrelease free of the hand lever 29 con sists in a simple movablecoupling. pawl 28 formed to function simultaneously as an armaturemovement of handle 29.

for the magnet pole 2'7 and as a mechanical connection between the handlever 29 and the oscillating armature 21. Thus in the case of excesscurrent the pawl 28 is influenced by the same magnet poles 27 whichattract the oscillating armature 21. e

In order to beable to maintain the switch as narrow as possiblethroughout its entire height, it is desirable to arrange the over centerspring 8, recording to Fig. 4, between the divided bi-metallic strip. Inlike manner, the oscillating armature 21 may be divided at the upperpart or provided with a notch 30 in order to provide room for the turnsof the spring. The force of the tilting spring 8 is used, according tothe invention, for securing the coupling member 31 in multi-polarswitches. For this purpose the spring tension adjusting screws 10 of theindividual switch poles are located below the cross-bar 31 and press thelatter against the individual oscillating armature 21.

In many cases it is desirable to increase the releasing currentstrength, for example during the starting of motors. In the present casethis can be very easily done by having the releasing armature 28connected to the operating handle 29 so that the air gap betweenreleasing armature 28 and pole 2'7 is enlarged by the switch closingAfter the completed closure of the switch the handle 29 is moved backinto the starting position by means of light spring 33 without therebyeffecting a release of the switch. The stop 32 is then so arranged thatin the closed position of the switch, the member 28 is moved the maximumdistance away from the magnetic poles 27 and thus increases the value ofcurrent required for releasing the switch.

An increase of the releasing current is also brought about for thethermo element when the switch closing lever 29 is moved entirely intothe end position against the pressure of the resilient stop'18, becauseZ0 is increased thereby. In the case of immediate reclosure of theswitch, after the release has been effected by the thermal element,however, this method is only possible once because with'the secondswitching-in the bi-metallic strip has become heated to a greaterextent. This limitation, however, is desirable for the protection of theswitch as well as for the protection of the circuit.

A shunting of current from the main contact spring 6 is effected in thefollowing manner: To

obtain relatively great flexibility of the leaf spring '6, the same mustbe kept somewhat long and thin,

from which a high electrical resistance results. According to theinvention now the non-oxidizing part 3 of the contact 2 is so arranged,for example, so extended, that it engages directly with thegrooved-shaped contact holder 34, as shown in Fig. 5. For currents whichcome below the limit current a large'part of the current goes from thecontact piece 3 direct through the contact holder 34 to the connectingbolt 35, and thus reduces perceptibly the currents passing throughspring 6. This is due to the fact that the ohmic resistance of thecontact holder 34 is small compared with the resistance of the resilientspring 6. The contact surfaces on the contact holder 34 may be providedwith a coating or a layer of a non-oxidizing metal.

As I have already stated, a modern switch must be capable of generaluse. The applications of my inventions have already been described inconnection with remote controlled switches, with beyond the point ofequilibrium of the movable system, and in place of the switch closingcoil, an undervoltage coil is provided. In this case the magnetism whichoriginates fromthe undervoltage coil, acts in opposition to the force ofthe auxiliary spring 24. The value of the releasing voltage can beconveniently adjusted by varying the tension of the spring 24. In orderto utilize the switch with undervoltage release for remote control, aseries resistance is inserted in the energizing circuit of the holdingmagnet, which resistance is entirely or partly short-circuited by' theremote control contact maker. The magnet thereby develops thenecessary-surplus pull with a large air-gapto closethe switch. Theresistance prevents overheating of the coil during the time the switchis closed In the case of the hitherto known forms of overload switcheswith under-voltage release, such a remote control arrangement was out ofthe question, hecauseof the energy requirements for closing the switch.I

For certain switching purposes, remote control switches, in many cases,must be equipped with an automatic arrangement for deenergizing theoperating coils after the operation has been effected. The interrupterin this case entails special difiiculties for the switch closing magnet,because the movement of the switch available for the movement of thecoil deenergizing interrupter is mostly very limited. For example, .inthe case of latched switches, the interruption can only be effected atthe very last moment, after the holdingpawls, etc. have come intoengagement. In order to make the coil deenergizing interrupter, or theinterlock contacts, safe in its action, special locking devices,quick-acting circuit arrangements, and complicated rod systems arenecessary, which frequently give rise to faulty operations. Anotherdifficulty is found in the fact that the switch closing magnet consumesa large amount of energy which results in considerable arcing when theinterlock contacts are opened. 7

In accordance with the present invention these difiiculties do notarise, since the forces necessary for the closing, as well as for theopening of the switch, are particularly small.- The energy consumptionof the switch closing coil is conse quently very small; also by means ofthe present construction, the described difiiculty caused by the limitedmovement available only at the last moment, is avoided. In this case,the entire switching movement, from the dead center position as far asthe final closing position is available for the interruption of theswitch closing coil currents. Therefore, as indicated in Fig. 5, theinterrupter contacts 38 and 39 for the switch closing coils can, withoutdifficulty, be coupled with the counter contact 4. Thus in case theoscillating armature has'fallen away, then the'main contactautomatically follows, and since the closing coil interrupter is moveddirect by the latter, the interruption of the coil circuit is likewiseguaranteed.

The interruption of the closing coil circuit by the arc. The countercontact 4 is located, according to Fig. 5, in a U-shaped recess 34 whichis tightly shut off above the contact by means of a short pole horn 5(Fig. 1). At the lower end and in front the recess is closed by means ofinsulating plates 36, 37 (Fig. .1).- With. the movement of the countercontact 4 a compression or a vacuum arises in the'recess, since theresilient spring 6 operates in the recess like a piston (Fig. 5). Theinterruption contacts 38 and 39 are arranged laterally at this recessopposite openings 40 which lead to the interior of the recess. Thus acurrent of air is blown against the interruption contacts for the switchclosing coil, or sucked up for the interruption contacts of the switchopening coil, whereby an easier extinction of the interruption arcs isbrought about.

The switch can also be used as a contactor. For this purpose it is onlynecessary to make the above-mentioned under-voltage arrangement and toleave out the series resistance. With the excitation of the switchclosing coil 22 the contactor is closed and is automaticallydisconnected by the spring 24 upon the coil 22 becoming deenergized.

The described construction can also be advantageously used when athermal quick-acting release is not desirable or is unnecessary. In thiscase the bi-metallic strip 1 is replaced by the usual metal (steel orbronze). On account of the easy operation of the switch it is possiblewithout difiiculty to adjust the releasing current for low values. Inthis manner, that is to say, without thermal release, the third orfourth pole of a multi-polar switch for example can be constructed.

The described magnet system can be used for protection against reversecurrent flow. Its operation is independent of voltage fluctuationswithin widelimits or of the magnitude of the current flow in the normaldirection. For this purpose a current coil 41 (Fig. 6) is wound aboutthe oscillating armature 21, while the shunt coils 22, 23, are connectedto the system or to a separate current source. The magnet systemtherefore acts as the-polarized magnet, that is to say, with normalcurrent flow, the armature is firmly attracted while with reversecurrent flow the release takes place upon the reversal of the current.The character of the magnet system can be differently arranged bychanging the airgaps in the left or right magnet circuit.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In combination, a thermo-responsive movable switch member havingmeans for insuring operation thereof with a snap action after movementof the switch member to a predetermined critical position, a cooperatingswitch member biased for movement with said thermo-responsive switchmember, and means for arresting the movement of said cooperating switchmember as soon as the snap action movement of said firstmentioned switchmember is initiated.

2. In combination, a movable thermo-responsive element, an over-centerspring connected to said element, a movable switch contact mounted onsaid element, a cooperating switch contact biased for movement with saidfirst contact, said thermo-responsive element being arranged to controlthe operation of said over-center spring to operate said movable switchcontact with a snap action to and from a closed circuit position withsaid cooperating contact, and means for arresting the movement of saidcooperating switch contact as soonas said over-center spring initiatesthe snap action movement of said movable contact from a closed circuitposition to an open circuit position.

3. An overload circuit breaker comprising a current conductingbimetallic element fixed atone end, a contact mounted upon the free endof said element, an over-center spring for operating said contact with asnap action between predetermined positions, said bimetallic elementbeing arranged to operate said over-center spring from one over-centerposition to the other overcen-' ter position, a cooperating contact forengaging said first-mentioned. contact, means for biasing said secondcontact for movement between predetermined positionsso that when saidbimetallic element operates the over-center spring said contacts remainin engagement until after said over-center spring initiates the openingof said contacts with said snap action.

4. An overload circuit breaker comprising a thermo-responsive movableswitch member, a co-' operating switch member biased for movement withthe thermo-responsive switch member to a predetermined position, and anelectromagnet energized in accordance with the current through thecontacts for electromagnetically holding the contacts in engagementduring movement thereof to said position.

5. In combination, a pair of cooperating movable switch members, a stopfor limiting the movement of one of said members, and anelectro-responsive means energized in accordance with the currentthrough the switch members for maintaining the same in engagement duringthe limited movement of said one member.

6. An overload circuit breaker comprising a current conductingbi-metallic element fixed at one end, a contact mounted upon the freeend of the element, a cooperating contact for engaging with the firstcontact when the temperature of the bi-r'netallic element is below apredetermined value and biased for movement with the first contact to apredetermined position upon the heating of the bi-metallic element abovesaid temperature, an over-center spring for biasing the first contactinto engagement with said c0- operating contact on one side of saidposition and out of engagement therewith on the other side of saidposition, and means for controlling the action of said over-centerspring to return said first contact into engagement with saidcooperating contact.

7. An over-load circuit breaker comprising a current conductingbi-metallic element fixed at one end, a contact mounted upon the freeend of the element, a cooperating contact for engaging with the firstcontact, an over-center spring for biasing the first contact intoengagement with said cooperating contact to conduct current to thebi-metallic element and out of engagement therewith upon a predeterminedmovement of the bi-metallic element due to heating, and means associatedwith said over-center spring for selectively biasing the first contactinto and out of engagement with said cooperating contact upon thesubsequent cooling of said oi-metallic element.

8. In combination, a thermal responsive element having one end fixed, anovercenter spring for biasing the free end of the element for movementon either side of a predetermined critical position depending upon thetemperature of the element, and means including a movable member forcontrolling the overcenter biasing spring to bias the free end of thethermal element from one side of said critical position to the otherindependently of the temperature of the element.

9. In combination, a thermal responsive element having an overcenterbiasing spring and means including a movable member for controlling theovercenter biasing spring to operate the thermal elementovercenterindependently of the temperature oi tble element.

10. In combination, a movable thermal responsive element having anovercenter biasing spring and electro-responsive means for controllingthe overcenter biasing spring to operate the thermal elementindependently of the temperature thereof.

11. In combination, a Uj-shaped thermal element having the ends thereoffixed, an overcenter spring having one end connected to the thermalelement and extending between the fixed ends thereof, and a movablemember connected to the other end of the overcenter spring forcontrolling the biasing action of the spring on the thermal element.

12. An overload circuit breaker comprising a thermo-responsive movableswitch member, an over-center biasing spring operatively associated withsaid member, means including a voltage responsive coil for controllingthe over-center biasing spring to operate said movable memberindependently of the temperature of said thermoresponsive member.

13. In combination, a movable thermo-respon-' sive element having anover-center biasing spring, means for controlling the over-centerbiasing spring to operate the thermal element independently of thetemperature thereof, comprising an electromagnet provided with anarmature, and energizing means associated with said armature operatingsaid armature between predetermined positions so as to control theoperation of said over-center biasing spring.

14. A circuit breaker comprising a current conducting bi-metallicelement fixed at one end, a contact mounted upon the free end of saidelement, a cooperating contact for engaging said first contact, anover-center spring attached at one end to said bi-metallic element forbiasing said first contact into and out of engagement with saidcooperating contact upon a predetermined movement of said bi-metallicelement, and a movable member to which the other end of said biasingspring is attached for'operating said biasing spring over-centerindependently of the temperature of said element.

15. A circuit breaker comprising a current conducting bi-metallicelement fixed at one end, a contact mounted upon the free end of saidelement, a cooperating contact for engaging said first contact, anover-center spring attached at one end to said bi-metallic element forbiasing said first contact into engagement with said 00- 100 operatingcontact to conduct current to the bimetallic element and out ofengagement therewith upon a predetermined movement of said bi-metallicelement, due to heating, and a movable member to which the outer end ofsaid bias- 105 ing spring is attached and electromagnetic means foroperating said movable member in one direction or the other to controlsaid over-center biasing spring so as to operate said elementindependently of the temperature thereof.

WILHELM HoPP.

